1. Field of the Invention
The present invention relates to an automatic differential pressure control for a particle flowthough apparatus which includes an electronic pressure monitoring feature, and more particularly, concerns a flow cytometer for determining one or more characteristics of particles flowing in a flow cytometer with such an automatic pressure control wherein the flow rate of ensheathed particles can be regulated and maintained at a preset level to facilitate the uniform operation of the apparatus. The method of using a technique to automatically control particle flow is also a part of the present invention.
2. Description of the Prior Art
There are a number of cell or particle analyzing devices using flow cytometer equipment and techniques which rely on hydrodynamically focused fluid flow through an analysis orifice where the specific characteristics of the flowing cells or particles can be determined. Flow analysis of particles has been used in the determination of the variety of characteristics of individual particles. This analysis is most useful in determining characteristics of cells for the collection of information which would aid in areas of research, hematology, immunology and the like. The researcher, for example, could be interested in determining specific characteristics of the individual cells where those cells need to be classified, identified, quantified and perhaps sorted for further investigations or analysis.
One commercially available flow cytometer which relies on a hydrodynamically focused fluid system is known as the FACScan.TM. instrument sold by the Becton Dickinson Immunocytometry Systems, Mt. View, Calif. The FACScan instrument rapidly analyzes cells on the basis of fluorescence and light scatter properties. Analysis is accomplished by introducing cells in suspension to the center of a focused liquid stream and causing them to pass, one at a time, through a focused light from a high power lamp or laser. Each cell is individually characterized by its light scatter signals and by the intensity and color of fluorescence emitted while it is illuminated.
In the aforementioned flow cytometer, a sheath liquid focuses the particles or cells as they pass through the orifice associated with the analyzing or counting capabilities. U.S. Pat. Nos. 4,503,385 and 4,526,276 describe particle analysis systems in which particles flowing in a stream are enveloped in a sheath liquid which focuses and confines the sample liquid (with the particles or cells) to the center of the flowing stream. U.S. Pat. No. 4,110,604 describes a particle density measuring system in which particles flowing in a stream are enveloped in a sheath liquid which focuses and confines the sample fluid (with the particles) to the center of the flowing stream.
Early systems provided for independent regulation of the air pressure used to drive the liquid with sample particles or cells from a supply test tube and of the air pressure in the sheathing liquid supply reservoir. The independent manual regulation and separate control of these two air pressures did not overcome the errors and disturbances produced by changes in liquid level in the sheathing liquid supply reservoir and pressure drop at the sheath liquid filter as it became partially obstructed with contamination or air. The air pressure regulator for the sheathing liquid supply and the other regulator for the sample test tube are required to be manually controlled. However, if reservoirs are depleted the air pressure generating the flow must be increased to maintain the proper flow rate. If the air pressure does not maintain the particle flow through the analysis orifice, the operation of the cytometer is impaired.
In the presently known and available particle flow-through equipment, electrically operated pumps, syringe pumps or the like are used in the fluidics of the system to move the liquid and particle flow through the flowcell analysis orifice and passageways. The Assignee of the present application has a co-pending application, U.S. Ser. No. 866,003 filed May 22, 1986, disclosing a housing for a flow cytometer apparatus with a particle unclogging feature The usual operation for these pumps is to force or draw liquid with particles from a sample test tube through a sample capillary tub centered in the sheathing liquid flowing in the direction of the particle analysis orifice. These syringe pumps used to aspirate and supply the sample to the analysis orifice tend to produce carryover, washout and other problems.
With the foregoing in mind, improved techniques for overcoming pressure differential variation in particle flow-through equipment are still being sought. Such improvements in a particle flow control should preferably be included in the particle flow-through apparatus so that the various parts thereof do not have to be constantly adjusted as the sheathing liquid reservoir is depleted. It is toward such an improvement that the present invention is directed.